1. Field of the Invention
The present invention relates to a solid oxide fuel cell and a manufacturing method thereof. More particularly, the present invention relates to a solid oxide fuel cell module which has a unit fuel cell module with an electrode and gas flow passages integral with each other and a plurality of the module stacked vertically and thus is easily manufacturable, economical, easily sealable and high in current density due to no need for a separate gas flow passage layer and a plurality of current collecting boards, a fuel cell system using the same and a manufacturing method thereof.
2. Description of the Related Art
A solid oxide fuel cell (hereinafter, “SOFC”) is highlighted as a third-generation fuel cell using a high efficiency low pollution electricity-generation method. The solid oxide fuel cell adopts thermo-chemically stable zirconia as an electrolyte with fuel and air electrodes attached thereto. The solid oxide fuel cell uses fuel gas such as hydrogen, methane or methanol without reformation and employs air or oxygen as an oxidant.
The SOFC utilizes zirconia as an electrolyte and yttria-stabilized zirconia (hereinafter, YSZ) to stabilize its crystalline structure of the electrolyte. This material exhibits an oxygen ion conductivity which is characteristically governed by temperature, and a desired conductivity for the fuel cell is attainable at a temperate of 900° C. to 1000° C. Therefore, the SOFC is typically operable at a temperature of 900° C. to 1000° C. and thus adopts ceramics for an electrode material to withstand such a high temperature. Generally, the fuel cell module is entirely made of ceramics.
However, an ion conductivity of the YSZ electrolyte is at most 0.1 S/cm even at a temperature of 1000° C. Thus, in manufacturing the fuel cell, a poreless and high-density thin film (10 μm to 30 μm) electrolyte layer should be coated to minimize YSZ-induced internal resistance.
A conventional planar SOFC utilizes an electrolyte plate as a support to coat front and back sides of the plate with air electrode and fuel electrode, respectively. In configuring the planar fuel cell module, fuel and air flow passages are formed in an interconnection connecting between the air and fuel electrodes. Such a planar fuel cell module should demonstrate sufficient mechanical strength to ensure the YSZ to serve as the support between the air and fuel electrodes. Notably, 8-YSZ (zirconia having itria added at an amount of 8 mol %) is weak in mechanical strength.
Accordingly, the planar SOFC needs to be thick in the support, i.e., the electrolyte layer. This increases voltage sag caused by internal resistance from inside the electrolyte, deteriorating capability of the fuel cell module. Moreover, the planar SOFC should be sealed in all edge portions thereof to prevent gas mixing in upper and lower parts of the cell.
Conventionally, glass was chiefly used as a sealing material. However, the glass material starts to soften from a temperature of 600° C. and thermal expansion during the subsequent temperature rise imposes strains between the respective fuel cell modules. This increases a risk of gas leakage, potentially damaging the fuel cell modules. Therefore, the glass material for sealing needs to be improved to be commercially viable.
To compensate for lacking mechanical strength of the planar fuel cell, a round tubular cell type is taught in U.S. Pat. Nos. 6,207,311 B1 and 6,248,468 B1.
Such a conventional round tubular cell is slightly inferior to the planar cell structure in terms of current density of a stack itself, but remarkably superior in terms of strength and gas sealing. This structure has air electrode, a solid electrolyte, fuel electrode and a current collecting layer stacked in their order on a porous support tube made of zirconia oxide, thereby forming a unit fuel cell.
Therefore, a gas sealing material is not required to be disposed between electrodes, thereby free from a problem associated with ceramic sealing which arises in the planar cell. Also, the fuel cell itself is of a robust ceramic structure with respective unit cells thereof formed on the secure support. Thus, the fuel cell is superbly resistant to thermal expansion. Moreover a metal interconnection can be employed owing to contact between the unit cells in a reducing atmosphere.
However, one of the fuel cell modules alone fails to boost capacity. The fuel cell modules are connected with each other in series or parallel to form a stack. Yet, the round tubular cell having the fuel cell modules connected with each other as just described, has a current path elongated to allow current generated to flow along a thin electrode surface, thereby potentially raising internal resistance of the entire fuel cell.
In addition, the fuel cell modules 1, when stacked, result in unnecessary spaces inside and outside the round tubular cell, thereby restricting current density per volume.
Recently, to overcome problems with the planar cell and round tubular cell type SOFCs, the fuel cell modules 1 feature both the planar cell structure and the round tubular cell structure, thereby solving a sealing problem of the planar cell. Furthermore, a flat tube type structure and a stack thereof are being developed to enhance current density as taught in Korean Patent Publication Application No. 10-2005-0021027, U.S. Pat. Nos. 6,416,897, and 6,429,051.
But the flat tube type structure, when stacked, also necessitates an electrical interconnection material for electrical and gas flow passages to enable flow of gases to the air or fuel electrodes. This increases mechanical strength of the stack and enlarges a contact area between the fuel cell modules to boost current density. Nonetheless, the metal interconnection characteristically may suffer thermal stress due to difference in thermal expansion with the ceramic fuel cell module when operating at a high temperature. Moreover, the need for using an interconnection material that has a good stability for thermal and electrical properties even during long operation at a high temperature increases price and also volume and weight of the stack.